Yolk is a source of maternal testosterone for developing birds.

TLDR

Bird development was traditionally thought to rely solely on embryonic or neonatal hormone production, yet variable hormone doses could explain individual behavioral differences in offspring. The study quantified yolk levels of multiple steroids—including testosterone—in freshly laid canary and zebra finch eggs using radioimmunoassay. Canary eggs contain higher yolk testosterone than zebra finch eggs, with levels rising across a clutch and originating maternally; this hormone correlates with juvenile social rank, indicating maternal testosterone can influence offspring aggression.

Abstract

The sex steroid hormones that affect development in birds have been thought to be produced exclusively by the embryo or neonate. I used radioimmunoassay to measure the amounts of androstenedione, 5 alpha-dihydrotestosterone, testosterone, 17 beta-estradiol, and corticosterone in the yolk of freshly laid canary (Serinus canaria) and zebra finch (Poephila guttata) eggs. Testosterone was found in both canary and zebra finch eggs, but its contents were much higher in the former than in the latter. The testosterone content of canary eggs in a same clutch increased with the order of laying, regardless of the genetic sex of the offspring that hatched from these eggs. Yolk testosterone was also present in the eggs of female canaries that were kept without a male, indicating that it is of maternal origin. The social rank of juvenile canaries was positively correlated with the concentration of yolk testosterone in the eggs from which they hatched, suggesting that the development of aggressive behavior of offspring might be subject to modification by maternal testosterone. These findings indicate that female songbirds can bestow upon their eggs a dose of hormone that modifies the behavior of offspring. Variable doses of these hormones might explain some of the individual variation in offspring behavior.